feat(pack): add checkhealth

Problem: No `:checkhealth` entry.

Solution: Add `:checkhealth vim.pack`. Design it around validating
  assumptions about `vim.pack` usage and giving actionable advice to fix
  possible issues. In particular:
  - Basic requirements are met: `git` executable, lockfile, and
    plugin directory are present.
  - Lockfile has correct shape (json with correct field types) and
    aligns with plugins on disk (revision and source are as expected).
  - Plugin directory has proper entries: all directories, in the state
    that `vim.pack` might leave them.
    Also notify if there are not active plugins. Those might be lazy
    loaded (okay) or removed an entry from the config without
    `vim.pack.del()` (not okay).

  Make sure that it passes with brief feedback if it doesn't look like
  `vim.pack` is being used.

  Duplicate check for `git` executable (on top of already existing
  very similar 'health/health.lua' check for "External tools") to have
  it shown after `:checkhealth vim.pack`. Keep the other check for more
  exposure and possibly for other plugins to reuse it.

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